Conventional reciprocating piston internal combustion engines are very complex mechanisms employing a great many parts which are subject to wear and which contribute to losses of efficiency due to friction. The reciprocating motion of the pistons cause vibrations which needlessly absorb input energy, create inertia and contribute to the failure of components of such engines and structures to which they are attached. While some improvements have been made in the effort to optimize the efficiency of piston engines, there is a fixed upper limit to the thermal efficiency of the piston engine concept which will forever limit its ability to realize more than a thermal efficiency of approximately thirty percent in production engines.
The thermal efficiency of an Otto cycle engine, such as internal combustion piston engines, is related to the expansion ratio of the engine, which is the ratio of the swept volume plus the head space volume of the cylinder to the head space volume. In a conventional piston engine, the expansion ratio is equal to the compression ratio of the engine, since compression and expansion occur in the same cylinder with the same volume sweeps by the piston. For a fuel with a given octane rating, the compression ratio must be limited to a value below which compression induced ignition or knock occurs too early in the compression cycle. In order to increase the thermal efficiency of a piston engine, it is necessary to increase the compression ratio, to thereby increase the expansion ratio, which requires a higher octane rating of the fuel at greater expense.
There are energy losses in piston engines due to the necessity of reversing the direction of linear motion of the pistons and the conversion of the linear motion to rotary motion. The expansion stroke of a piston engine occurs when the piston is just passing top dead center and when the piston rod and the crank throw of the crankshaft are almost aligned. Although the pressure within the cylinder is at a maximum at this time and the force available from the piston is greatest, the moment arm of the crankshaft throw is at a minimum. Thus, only a small component of piston force is available to the crankshaft to cause rotation. The remaining component of piston force is directed radially to the crankshaft and is wasted as heat to atmosphere. When the crankshaft is in an angular position such that the moment provided by the crank throw :s maximized, the cylinder pressure and the piston force resulting therefrom have diminished to less than half of that originally available resulting, with other inefficiencies of the piston engine concept, in unacceptably low thermal and mechanical efficiency.
In general, gas turbine engines, a form of rotary engine, have been applied most successfully to aircraft propulsion as fairly large engines. While a few small engines have been built and tested in passenger car size ground vehicles, the disadvantages of small gas turbine engines have outweighed the advantages. Maintenance of such engines is more expensive than for a comparable size piston engine. Additionally, the engine power relative to engine speed range is narrower for gas turbine engines whereby ground vehicles would require more complex and thus more expensive transmissions than are needed for piston engine driven cars.
The only well-known rotary engine which has achieved any degree of success in ground vehicles is the Wankel rotary internal combustion engine. But this engine is even less fuel efficient than the standard four cycle piston engine. The Wankel rotary engine has been applied with some success to smaller ground vehicles, such as automobiles and motorcycles. While the major component of motion of the Wankel rotor is, as its name implies, rotary, there is additionally a reciprocating component since the housing cavity in axial cross section is elongated rather than circularly cylindrical. The reciprocating component of rotor motion is a consequence of the geometry of the rotor and housing, the manner of gearing the rotor to the engine shaft, and the manner in which the fuel-air mixture is compressed and exhausted after combustion. This causes some loss of efficiency since energy must be expended in accelerating and decelerating the rotor through its linear components of motion. Additionally, Wankel engines have identical compression and expansion ratios such that the thermal efficiency of a Wankel engine is limited by the compression ratio possible with the fuel utilized, in the same manner as piston engines.